(1) Field of the Invention
The present invention relates to a wavelength-division multiplexing transmission system which wavelength-division-multiplexes optical signals. at a plurality of wavelengths and transmits the same, a method for designing a loss difference compensator for optical devices used in the wavelength-division multiplexing transmission system, and a method for configuring the wavelength-division multiplexing transmission system.
(2) Description of the Related Art
In order to realize a future multimedia network, there is, in these years, a demand for an optical communication system by which a larger capacity communication is feasible. A lot of researches have. been conducted on a time-division multiplexing (TDM) system, an optical time-division multiplexing (OTDM) system in the optical domain, a wavelength-division multiplexing (WDM) system and the like, as multiplexing (transmission) systems enabling very large capacity communications.
Among them, the WDM transmission system is expected as a means to realize a optical network which can flexibly multiplex different types of services by crossconnecting or branching and inserting at the optical level using a wide gain band of an erbium doped fiber amplifier (EDFA).
In the WDM transmission system (wavelength-division multiplexing transmission system), optical repeaters each having an optical amplifier such as an EDFA or the like are, in general, connected in plural stages according to a transmission power (a transmittable distance) of a used optical transmission line, whereby a long-distance transmission of a WDM signal is possible.
However, an optical amplifier such as an EDFA or the like has an amplification (gain) characteristic dependent on wavelength, as well known. For this reason, even if a WDM signal having a flat power at each wavelength is transmitted, there are generated differences in loss of power of the wavelengths (gain tilt) of the received WDM signal at a receiving end.
In a system using a single mode fiber (SMF) whose zero-dispersion characteristic is 1.3 xcexcm (micrometer) in a transmission line to WDM-transmit a lightwave centered at a 1.55 xcexcm band, for example, an optical signal at a shorter wavelength has a smaller power at the receiving end, since the optical amplifier has a characteristic that the longer the wavelength, the larger the gain in the used wavelength band is [for example, 1530 to 1570 nm (nanometer)] (tilt upward toward the right). In the worst case, it is impossible to receive the optical signal at a shorter wavelength.
For the purpose of equalizing (compensating) differences in loss of the wavelengths generated in the WDM signal due to the gain characteristic of the optical amplifier, there have been proposed various techniques.
A technique disclosed in Japanese Patent Laid-Open Publication No. 9-436-47, for example, flattens the gain characteristic of the optical amplifier (EDFA) using a bending loss caused by a bending of an optical fiber. Namely, taking advantage of a fact that when an optical fiber is bent, the longer the wavelength, the larger the bending loss is, an optical fiber having a transmission characteristic approximately reverse to the gain characteristic of the optical amplifier is added to flatten the gain characteristic of the optical amplifier.
However, this technique can yield only an optical filter having a linear transmission characteristic since the technique utilizes a bend of an optical fiber. The gain characteristic of an actual optical amplifier is not linear so that such an optical filter cannot effectively flatten the gain characteristic of the optical amplifier, of course.
There is also proposed a technique in which a Fabry-Perot etalon filter is used as an optical filter being able to accomplish a non-linear transmission characteristic to flatten the gain characteristic of the optical amplifier more effectively.
Meanwhile, an optical fiber (SMF) used in the present WDM transmission system has absorption losses inherent to silica glass that is a material of the optical fiber such as ultraviolet loss, infrared loss and the like, and scattering losses such as Rayleigh scattering and the like (refer to FIG. 26). A transmission loss characteristic 100 is generated dependent on, mainly, Rayleigh scattering and infrared absorption among these losses. FIG. 27 shows a 1.55 xcexcm band shown in FIG. 26 when enlarged. As seen from FIG. 27, a loss difference (a tilt downward to the right) are generated in a used wavelength band 101.
For this, even if a WDM signal having a flat power at each wavelength is inputted to an optical fiber, a difference is generated in the receiving level of the channel (a wavelength) at a receiving end as shown in FIG. 28, for example, due to the wavelength-dependent transmission loss characteristic of the optical fiber, leading to a variation in the transmission characteristic (S/N ratio) of each channel.
The above phenomenon is not a serious issue in the case where a used wavelength band is narrow (a WDM signal at one to several wavelengths, for example) and a transmission distance is short [several tens km (kilometer), for example] since the phenomenon can be avoided by setting the used wavelength band 101 shown in FIG. 27 in a portion where the transmission loss characteristic 100 is almost flat. However, in the case where the used wavelength band 101 is wide, a tilt inevitably exists in the transmission loss characteristic 100 so that a difference in loss is generated according to a transmission distance, leading to a very serious issue.
Particularly, a recent demand is to transmit more wavelengths (16 wavelengths or more, for example) for a longer distance (several hundreds kilometers, for example). For this, a loss difference generated due to the transmission loss characteristic 100 of the optical fiber is not negligible.
In the case where an SMF is used to perform optical transmission (WDM transmission) with a lightwave in a 1.55 xcexcm band, there is often used a technique in which a dispersion compensation fiber (DCF) is used to cancel dispersion generated in a WDM signal due to a dispersion characteristic of the SMF. However, since the DCF is an optical fiber whose basic material is silica glass similarly to the SMF, the DCF has a transmission loss characteristic similar to that of the SMF.
When the DCF is used, a wavelength-dependent loss difference similar to that of the SMF is generated in not only the SMF but also the DCF so that a larger difference is generated in S/N ratio of each channel, as compared with a case of only the SMF, due to the transmission loss characteristic of both of the SMF and the DCF.
In the case where a used wavelength band 101 for a WDM signal is 1530 to 1570 nm, for example, a tilt (a loss difference) of 0.4 dB is generated when the WDM signal is transmitted for 80 km if a loss difference in the used wavelength band of the SMF is 0.005 dB/km. Moreover, if the DCF is used to compensate dispersion degradation in the SMF, a DCF with xe2x88x921000 to xe2x88x921200 pb/nm is required for 80 km of the SMF. This corresponds to about 10 km of a fiber length, generating a loss:difference of about 1 dB.
When the SMF and DFC are connected in several stages (3 or 4 stages, for example) to linearly repeat, a loss difference of about 5 to 6 db in total is generated by the SMFs and the DCFs. As a result, an S/N ratio of each channel is largely degraded. In the worst case, it is impossible to receive a wavelength (a channel) on the shorter wavelength""s side.
When the SMF or the DCF is used in a WDM transmission system, it is important to compensate the above loss difference, in consideration of a wavelength-dependent transmission loss characteristic of the SMF or the DCF itself. Particularly, when 16 or more lightwaves are wavelength-division-multiplexed and a WDM signal whose used wavelength band is 12 nm or more is transmitted, the above compensation is very important since a tilt in the transmission loss characteristic appears noticeable.
However, in the above-described WDM transmission system, the principal object is to flatten the gain characteristic of an optical amplifier on the assumption that a loss difference in a WDM signal at the receiving end is generated due to mainly the wavelength-dependent gain characteristic of the optical amplifier, there is thus no consideration on a transmission loss characteristic of the SMF or the DCF. Accordingly, even if an output power of the WDM signal can be flat at each wavelength in the optical amplifier, a variation is inevitably generated in the transmission characteristic (S/N ratio) of each channel depending on a transmission distance on the SMF or the DCF thereafter.
In the light of the above problems, an object of the present: invention is to provide a wavelength-division multiplexing transmission system which can compensate a loss difference of an optical signal at each wavelength generated in a wavelength-division-multiplexed optical signal due to a wavelength-dependent transmission loss characteristic of not only an optical amplifier but also an optical transmission line and a dispersion compenstor to transmit the wavelength-division-multiplexed optical signal in a wide wavelength band for a long distance.
Another object of the present invention is to provide a method for designing a loss difference compensator for an optical device used in the wavelength-division multiplexing transmission system, thereby readily designing a loss difference compensator which can compensate a loss difference generated due to a loss characteristic of the optical device used in the wavelength-division multiplexing transmission system. Still another object of the present invention is to provide a method for configuring a wavelength-division multiplexing transmission system, using the loss difference compensator designed in the above designing method.
The present invention therefore provides a wavelength-division multiplexing transmission system comprising an optical transmission line for transmitting a wavelength-division-multiplexed optical signal having a wide wavelength band in which optical signals at a plurality of wavelengths are wavelength-division-multiplexed, and a loss difference compensator disposed in the optical transmission line. to compensate a loss difference of an optical signal at each wavelength generated in the wavelength-division-multiplexed optical signal due to a wavelength-dependent transmission loss characteristic of the optical transmission line.
The wavelength-division multiplexing transmission system of this invention can thereby make the loss characteristic of the optical transmission line be independent of wavelength to always suppress a variation in receiving level at each wavelength to a minimum. It is therefore possible to readily realize a system which can transmit a wavelength-division-multiplexed optical signal in a wide wavelength band for a long distance, thus largely improving serviceability of various communications using the wavelength-division multiplexing transmission.
The above loss difference noticeably appears when the above wavelength-division-multiplexed optical signal has a wavelength band of not less than 12 nanometer centered at a 1.55 micrometer band as the above wide wavelength band, and cannot be ignored depending on a transmission distance of the wavelength-division-multiplexed optical signal. In such case, the loss difference compensation by the above loss difference compensator is very effective.
The above loss difference compensator may be configured with an optical filter having a loss difference compensation characteristic according to a transmission distance of the wavelength-division-multiplexed optical signal in order to compensate a loss. difference generated according to the transmission. distance. In which case, it is possible to compensate the above loss difference only by disposing the optical filter in the optical transmission line without any special control for compensating the above loss difference, leading to simplification of the system.
The present invention further provides a wavelength-division multiplexing transmission system comprising an optical transmission line for transmitting a wavelength-division-multiplexed optical signal in which optical signals at a plurality of wavelengths are wavelength-division-multiplexed, a dispersion compensator for compensating dispersion generated in the wavelength-division-multiplexed optical signal due to a dispersion characteristic of the optical transmission line, and a loss difference compensator disposed in the optical transmission line to compensate a loss difference of an optical signal at each wavelength generated in the wavelength-division-multiplexed optical signal due to a wavelength-dependent transmission loss characteristic of the optical transmission line and a wavelength-dependent transmission loss characteristic of the dispersion compensator.
The above wavelength-division multiplexing transmission system can make the loss characteristic of the whole system be wavelength-independent. Even in a system of a type compensating dispersion of an optical transmission line using a loss difference compensator, it is possible to always suppress a variation in receiving level at each wavelength to a minimum. Accordingly, a WDM signal can be transmitted for a long distance in a high signal quality, and serviceability of various communications using the wavelength-division multiplexing transmission can be largely improved.
The above loss difference noticeably appears when the above wavelength-division-multiplexed optical signal has a wide wavelength band wider than a predetermined wavelength band (12 nanometer centered at a 1.55 micrometer band, for example), and cannot be ignored depending on a transmission distance of the wavelength-division-multiplexed optical signal. In such case, loss difference compensation by the loss difference compensator is very effective.
Incidentally, if the above loss difference compensator is disposed in the pre-stage of the dispersion compensator, it is possible to suppress a level of the wavelength-division-multiplexed optical signal in the pre-stage of the dispersion compensator. In this case, a phenomenon that an adverse effect is exerted on the above dispersion compensation because of an excessively high input level of the wavelength-division-multiplexed optical signal to the dispersion compensator can be avoided. It is therefore possible to stably perform the dispersion compensation and the loss difference compensation on the wavelength-division-multiplexed optical signal, leading to a large improvement of reliability of the system.
The. above loss difference compensator may be configured with an optical filter having a loss difference characteristic according to a transmission distance of the above wavelength-division-multiplexed optical signal in order to compensate a loss difference generated according to the above transmission distance. In which case, it is possible to realize compensation of a loss difference generated by the above optical transmission line and the dispersion compensator only by providing the optical filter in the optical transmission line, which leads to simplification of the system.
The present invention still further provides a wavelength-division multiplexing transmission system comprising an optical transmission line for transmitting a wavelength-division-multiplexed optical signal in which optical signals at a plurality of wavelengths are wavelength-division-multiplexed, an optical amplifier for amplifying the wavelength-division-multiplexed optical signal, and a loss difference compensator disposed in the optical transmission line to compensate a loss difference of an optical signal at each wavelength generated in the wavelength-division-multiplexed optical signal due to a wavelength-dependent transmission loss characteristic of the optical transmission line and a wavelength-dependent amplification loss characteristic of the optical amplifier.
In this case, the loss characteristic of the whole system can be wavelength-independent. Even in a system transmitting a wavelength-division-multiplexed optical signal while appropriately amplifying the same, it is possible to always suppress a variation in receiving level at each wavelength to a minimum, which leads to a large improvement of serviceability of various communications using wavelength-division multiplexing transmission.
The above loss difference noticeably appears when the above wavelength-division-multiplexed optical signal has a wide wavelength band wider than a predetermined wavelength band (12 nanometer centered at a 1.55 micrometer band, for example), and cannot be ignored depending on a transmission distance of the wavelength-division-multiplexed optical signal. In such case, loss difference compensation by the above loss difference compensator is very effective.
The above. loss difference compensator may be configured with an optical filter having a loss difference compensation characteristic according to a transmission distance of the above wavelength-division-multiplexed optical signal in order to compensate the above loss difference generated according to the transmission distance. In which case, it is possible to readily realize compensation of a loss difference generated by the above optical transmission line and the optical amplifier only by disposing the optical filter in the optical transmission line without any special control for the above loss difference compensation.
The present invention still further provides a wavelength-division multiplexing transmission system comprising an optical transmission line for transmitting a wavelength-division-multiplexed optical signal in which optical signals at a plurality of wavelengths are wavelength-division-multiplexed, a dispersion compensator for compensating dispersion generated in the wavelength-division-multiplexed optical signal due to a dispersion characteristic of the optical transmission line, an optical amplifier for amplifying the wavelength-division-multiplexed optical signal, and a loss difference compensator disposed in the optical transmission line to compensate a loss difference of an optical signal at each wavelength generated in the wavelength-division-multiplexed optical signal due to a wavelength-dependent transmission loss characteristic of the optical transmission line, a wavelength-dependent transmission loss characteristic of the dispersion compensator and a wavelength-dependent amplification loss characteristic of the optical amplifier.
In this case, the loss characteristic of the whole system can be wavelength-independent. Even in a system transmitting a wavelength-division-multiplexed optical signal while appropriately amplifying the same, it is possible to always suppress a variation in receiving level at each wavelength to a minimum so that the wavelength-division-multiplexed optical signal can be transmitted for a longer distance in a higher signal quality, which leads to a large improvement of serviceability of various communications using wavelength-division multiplexing transmission.
The above loss difference noticeably appears when the above wavelength-division-multiplexed optical signal has a wide wavelength band wider than a predetermined wavelength band (particularly, 12 nanometer centered at a 1.55 micrometer band), and cannot be ignored depending on a transmission distance of the wavelength-division-multiplexed optical signal. In such case, loss difference compensation by the above loss difference compensator is very effective.
In this case, if the above loss difference copensator is disposed in the pre-stage of the dispersion compensator, it is possible to suppress a level of the wavelength-division-multiplexed optical signal in the pre-stage (loss difference compensator) of the dispersion compensator. It is therefore possible to avoid an adverse effect on the above dispersion compensation because of an excessively high input level of the wavelength-division-multiplexed optical signal to the dispersion compensator without an exclusive circuit for suppressing a level of the wavelength-division-multiplexed optical signal.
The above loss difference compensator may be configured with an optical filter having a loss difference compensation characteristic according to a transmission distance of the above wavelength-division-multiplexed optical signal in order to compensate a loss difference generated according to the transmission distance. In which case, it is possible to very easily accomplish compensation of a loss difference caused by the optical transmission line, the dispersion compensator and the optical amplifier only by disposing the optical filter in the optical transmission line without any special control for compensating the above loss difference.
The present invention also provides a method for designing a loss difference compensator for an optical device used in a wavelength-division multiplexing transmission system comprising the steps of preparing a menu table in which, correspondingly to information on the optical device used in the wavelength-division multiplexing transmission system for transmitting a wavelength-division-multiplexed optical signal in which optical signals at a plurality of wavelengths are wavelength-division-multiplexed, information on a loss difference compensation characteristic of a loss difference compensator for compensating a loss difference of an optical signal at each wavelength generated in the wavelength-division-multiplexed optical signal due to a wavelength-dependent loss characteristic of the optical device is classified, and selecting a loss difference compensation characteristic corresponding to the optical device used in the wavelength-division multiplexing transmission system on the basis of the menu table to design a loss difference compensator having the loss difference compensation characteristic.
According to the above designing method, it is possible to suppress the number of types of the loss difference compensator having the most suitable loss difference compensation characteristic according to a used optical device. Therefore, a loss difference compensator according to a used optical device may be fabricated in large volumes, and thus a manufacturing cost of the loss difference compensator may be reduced, which leads to a decrease of a cost of the wavelength-division multiplexing transmission system to be configured.
If the above optical device is an optical transmission line, information on the loss difference compensation characteristic compensating a loss difference generated due to a wavelength-dependent transmission loss characteristic of the optical transmission line is classified correspondingly to information on the optical transmission line, thereby readily design a loss difference compensator having the most suitable loss difference compensation characteristic according to a used optical transmission line.
If the above optical device is an optical transmission line and a dispersion compensator disposed in the optical transmission line to compensate dispersion generated in the wavelength-division-multiplexed optical signal due to a dispersion characteristic of the optical transmission line, information on a loss difference compensation characteristic compensating the loss difference generated due to a wavelength-dependent transmission loss characteristic of the optical transmission line and a wavelength-dependent transmission loss characteristic of the dispersion compensator is classified correspondingly to a combination of information on the optical transmission line and information on the dispersion compensator, thereby readily designing a loss difference compensator having the most suitable loss difference compensation characteristic according to a used optical transmission line and dispersion compensator.
If the above optical device is an optical transmission line, and an optical amplifier disposed in the optical transmission line to amplify the wavelength-division-multiplexed optical signal, information on a loss difference compensation characteristic compensating a loss difference generated due to a wavelength-dependent transmission loss characteristic of the optical transmission line and a wavelength-dependent amplification loss characteristic of the optical amplifier is classified, correspondingly to a combination of information on the optical transmission line and information on the optical amplifier, thereby readily designing a loss difference compensator having the most suitable loss difference compensation characteristic according to a used optical transmission line and optical amplifier.
If the above optical device is an optical transmission line, a dispersion compensator disposed in the optical transmission line to compensate dispersion generated in the wavelength-division-multiplexed optical signal due to a dispersion characteristic of the optical transmission line, and an optical amplifier disposed in the optical transmission line to amplify the wavelength-division-multiplexed optical signal, information on a loss difference compensation characteristic compensating a loss difference generated due to a wavelength-dependent loss characteristic of each of the optical transmission line, the dispersion compensator and the optical amplifier is classified, correspondingly to a combination of information on the optical transmission line, information on the dispersion compensator and information on the optical amplifier, thereby readily designing a loss difference compensator having the most suitable loss difference compensation characteristic according to used optical transmission line, dispersion compensator and optical amplifier.
The above loss difference noticeably appears when the above wavelength-division-multiplexed optical signal has a wide wavelength band wider than a predetermined wavelength band (particularly, 12 nanometer centered at a 1.55 micrometer band), and cannot be ignored depending on a transmission distance of the wavelength-division-multiplexed optical signal. In such case, the loss difference compensator designed as above is very effective.
In the above menu table, the information on the above optical device and the loss difference compensation characteristic may be classified according to a transmission distance of the above wavelength-division-multiplexed optical signal. In which case, a loss difference compensation characteristic corresponding to the optical devices according to a transmission distance of the wavelength-division-multiplexed optical signal is selected on the basis of the menu table.
It is thereby possible to suppress the number of types of the loss difference compensation characteristic to a minimum according to a necessary transmission distance, leading to a decrease of the number of types of the loss difference compensator to be designed, and mass production of the loss difference compensator in larger volumes.
The present invention still further provides a method for configuring a wavelength-division-multiplexing transmission system comprising the steps of preparing a menu table in which information on a loss difference compensation characteristic of a loss difference compensator for compensating a loss difference of an optical signal at each wavelength generated in a wavelength-division-multiplexed optical signal in which- optical signals at a plurality of wavelengths are wavelength-division multiplexed due to a wavelength-dependent loss characteristic of an optical device used in the wavelength-division-multiplexing transmission system for transmitting the wavelength-division multiplexed optical signal is classified correspondingly to information on the optical device, selecting a loss difference compensation characteristic corresponding to the optical device used in the wavelength-division multiplexing transmission system on the basis of the menu table to design a loss difference compensator having the loss difference compensation characteristic, and configuring the wavelength-division multiplexing transmission system using the optical device and the designed loss difference compensator.
According. to the method for configuring a wavelength-division multiplexing transmission system of the present invention, it is possible to configure a wavelength-division multiplexing transmission system which can compensate a loss difference of an optical signal at each wavelength generated by the used optical divice, very readily and at a low cost.